Re: [Qemu-devel] [Qemu-block] [PATCH v3 01/10] qapi: Add transaction support to block-dirty-bitmap operations

[Stefan Hajnoczi]

On Thu, May 07, 2015 at 01:22:26PM -0400, John Snow wrote:
> 
> 
> On 05/07/2015 10:54 AM, Stefan Hajnoczi wrote:
> > On Wed, Apr 22, 2015 at 08:04:44PM -0400, John Snow wrote:
> >> +static void block_dirty_bitmap_clear_prepare(BlkTransactionState
> >> *common, +                                             Error
> >> **errp) +{ +    BlockDirtyBitmapState *state =
> >> DO_UPCAST(BlockDirtyBitmapState, +
> >> common, common); +    BlockDirtyBitmap *action; + +    action =
> >> common->action->block_dirty_bitmap_clear; +    state->bitmap =
> >> block_dirty_bitmap_lookup(action->node, +
> >> action->name, +
> >> &state->bs, +
> >> &state->aio_context, +
> >> errp); +    if (!state->bitmap) { +        return; +    } + +
> >> if (bdrv_dirty_bitmap_frozen(state->bitmap)) { +
> >> error_setg(errp, "Cannot modify a frozen bitmap"); +
> >> return; +    } else if
> >> (!bdrv_dirty_bitmap_enabled(state->bitmap)) { +
> >> error_setg(errp, "Cannot clear a disabled bitmap"); +
> >> return; +    } + +    /* AioContext is released in .clean() */ 
> >> +} + +static void
> >> block_dirty_bitmap_clear_commit(BlkTransactionState *common) +{ +
> >> BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState, +
> >> common, common); +    bdrv_clear_dirty_bitmap(state->bitmap); +}
> > 
> > These semantics don't work in this example:
> > 
> > [block-dirty-bitmap-clear, drive-backup]
> > 
> > Since drive-backup starts the blockjob in .prepare() but 
> > block-dirty-bitmap-clear only clears the bitmap in .commit() the
> > order is wrong.
> > 
> > .prepare() has to do something non-destructive, like stashing away
> > the HBitmap and replacing it with an empty one.  Then .commit() can
> > discard the old bitmap while .abort() can move the old bitmap back
> > to undo the operation.
> > 
> > Stefan
> > 
> 
> Hmm, that's sort of gross. That means that any transactional command
> *ever* destined to be used with drive-backup in any conceivable way
> needs to move a lot more of its action forward to .prepare().
> 
> That sort of defeats the premise of .prepare() and .commit(), no? And
> all because drive-backup jumped the gun.

No it doesn't.  Actions have to appear atomic to the qmp_transaction
caller.  Both approaches achieve that so they are both correct in
isolation.

The ambiguity is whether "commit the changes" for .commit() means
"changes take effect" or "discard stashed state, making undo
impossible".

I think the "discard stashed state, making undo impossible"
interpretation is good because .commit() is not allowed to fail.  That
function should only do things that never fail.

> That's going to get hard to maintain as we add more transactions.

Yes, we need to be consistent and stick to one of the interpretations in
order to guarantee ordering.

Unfortunately, there is already an inconsistency:

1. internal_snapshot - snapshot taken in .prepare()
2. external_snapshot - BDS node appended in .commit()
3. drive_backup - block job started in .prepare()
4. blockdev_backup - block job started in .prepare()

external_snapshot followed by internal_snapshot acts like the reverse
ordering!

Stefan

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